Delivery of compositions to a target cell or tissue faces various transport barriers. Nucleic acids that encode gene products, such as proteins, and non-coding RNA (e.g., siRNAs) can be delivered directly to a desired vertebrate subject, or can be delivered ex vivo to cells obtained or derived from the subject, and the cells can be re-implanted into the subject. Delivery of such nucleic acids to a vertebrate subject is desirable for many purposes, such as, for gene therapy, to induce an immune response against an encoded polypeptide, or to regulate the expression of endogenous genes. The use of this approach has been hindered because free DNA is not readily taken up by cells and free RNA is rapidly degraded in vivo. Moreover, delivery can also be problematic. For instance, subcutaneous or intramuscular injections using hypodermic needles can cause pain, trauma, and anxiety in a subject.
Delivery of one or more polypeptides, whether directly as protein or indirectly by an encoding polynucleotide, has many useful applications, including vaccination. Vaccination has proven an effective means to fight and even eradicate infectious diseases. The influenza vaccine, for example, is currently recommended by the CDC as the primary method for preventing influenza. However, influenza virus has a high rate of mutation and antigenic variation and a new vaccine is typically produced each year based upon the predicted circulating pathogenic strains. This poses a number of challenges. For instance, the effectiveness of the vaccine is only as good as the prediction. If the prediction of the dominant strain is incorrect, the vaccine will have limited effectiveness for most people. Further, it can take months to produce enough influenza vaccine to vaccinate a population.